1. Field of the Invention
The present invention relates to an emission control system for an asphalt kettle, and in particular, to an emission control system that includes a separate afterburner device housing a high powered fan which pushes fumes into a combustion chamber.
2. Description of the Related Art
Melting kettles for heating and melting bituminous materials of the asphaltic tar type are well known and have been used for many years in the roofing and roadway surfacing industries. Generally, these kettles are self-contained and transportable to the job site. As such, it is necessary that they do not require any undue servicing and that they be uncomplicated and reliable in operation.
The bituminous materials heated in these kettles inherently include compounds that vaporize and form objectionable fumes, smoke, and odors which have become an increasing nuisance in recent times. This problem is aggravated when the bituminous melt is heated to the higher temperatures commonly employed today, such that hydrocarbons of a higher order or length are vaporized.
Conventionally, fumes from the asphalt have been vented directly into the atmosphere without any smog or smoke control. Starting in the 1970's, there was a major effort to reduce kettle fume emissions in response to strict air pollution standards imposed by air quality management districts in the San Francisco Bay Area and in Southern California. A variety of technologies for controlling fume emissions were tested, and in some cases kettles equipped with these controls were manufactured and placed into service in the field. In general the experience to date with these kinds of controls has been decidedly unsatisfactory, particularly from a safety standpoint.
The kettle innovations tried in the past involved two types of controls: (1) emission capture and destruction devices, and (2) load insertion devices. Unfortunately, both approaches were abandoned after the researchers and engineers involved in this effort concluded that certain characteristics inherent in each of these types of controls substantially increased both the likelihood and the severity of kettle explosions and fires, and therefore presented serious worker safety hazards.
Emission capture and destruction controls consist of a vent or exhaust system that evacuates fumes from the head space inside the kettle to a capture or destruction device. Past experience with such systems suggests that it is difficult to evacuate fumes from the interior headspace of the kettle without creating an explosion and fire risk. This is because kettle fumes contain a variety of chemicals that are highly flammable, and thus the vapor inside the kettle is potentially explosive. As with any potentially explosive vapor, kettle fumes have an upper and lower explosive limit. At concentrations of fume below the lower explosive limit ("LEL"), the mixture is too "lean" to burn, while at concentrations above the upper explosive limit ("UEL"), the mixture is too "rich" to burn. During kettle operations, the concentration of fumes typically drops below the LEL almost instantly whenever the hood is opened and fumes are evacuated. When the kettle hood is closed, the fume concentrations build rapidly and the mixture almost immediately becomes too "rich" to pose an explosion hazard.
The treatment or destruction systems tested included afterburners, reburners, filters, and condensation systems. These types of emission control systems must draw fumes from the headspace inside the kettle, and thus reduce the concentration of asphalt fumes inside the headspace. In the absence of a control that regulates the air flow to maintain a specific fume concentration, emission control systems may bring the fume concentration back down into the explosive range. If fume concentrations in the headspace are within the explosive range, an explosion will be triggered by either a spark or by autoignition if the temperature rises to between approximately 500 and 575.degree. F.
The kettle environment presents a variety of potential ignition sources, and previous attempts to suppress these ignition sources have generally been unsuccessful in controlling explosion hazards. For example, afterburner systems use an open flame that can act as an ignition source. The ignition risk associated with the afterburner can be reduced by the use of flame arresters, but previous experience indicates that flame arresters (at least those of past design) are prone to clogging and have not been sufficiently reliable to work effectively under actual operating conditions.
Another problem identified in connection with emission control is accumulation of coke or carbon deposits in the melting chamber of the kettle. When a kettle is operated at high temperatures, this buildup can become so hot that it will glow red, and act as an additional ignition source.
In systems that do not use afterburners or reburners, other potential ignition sources, such as the heating tube vent stack or flue, are present in virtually all kettles. Kettle heating tubes generally run lengthwise through the vessel, then turn and pass vertically through the kettle headspace to vent above the top deck of the kettle. Temperatures at hot spots on the heating tubes frequently exceed 500-575.degree. F. on the surface of the vent stack, providing an ignition source.
Finally, in some situations the asphalt itself can become an ignition source. Some roofers overheat the asphalt for a variety of reasons even though this practice has long been strongly discouraged. If the asphalt reaches temperatures in the range of 500-575.degree. F., the surface of the hot asphalt itself can become an ignition source and trigger a kettle explosion.
The second approach taken in reducing emissions relates to the manner by which loads are inserted into the kettle. Loading devices that permit refilling of the kettle without opening the lid substantially reduce fume emissions. A variety of designs have been tested and marketed, including "mail slot" openings, and loading "arms" that rotate and drop the solid asphalt into the melting chamber of the kettle. Since all of these devices must be located above the liquid level in the melting chamber, they increase the headspace above the liquid asphalt. This increased volume does not necessarily increase the risk of explosion, but can significantly increase the size of an explosion.
In addition, these loading devices can easily become clogged and difficult to use over time. This will effectively reduce the usable capacity of the kettle, creating an incentive to overheat the asphalt as described above.
U.S. Pat. No. 4,033,328, issued Jul. 5, 1977 and entitled "Tar Melting Kettle," discloses an asphalt kettle which includes a loading drum in the form of a rotating cylinder. Rotating the cylinder to align it with an opening in the melting chamber ensures that the contents of the kettle are not exposed to the atmosphere during loading.
This approach suffers from a number of problems. The user is denied visual access to the melting chamber to observe homogeneity of melting and the fluid level of the melted asphalt. This is dangerous insofar as the user may fail to detect that the fluid level has become dangerously low and cause a kettle fire.
In addition, the user is denied physical access to the melting chamber. This is dangerous insofar as coke and carbon residue is typically removed by skimming the top of the melt. Failure to adequately remove the coke and carbon in this manner can lead to the buildup of deposits and create an ignition source as described above.
California regulatory bodies driving the effort to develop emission control technologies ultimately determined that air pollution requirements would best be met through adoption of work practices which avoid overheating, and minimize the amount of time the kettle hood is left open. However, efforts to develop feasible, practical and safe emission controls for kettles have continued despite the unsatisfactory past experience with such systems.
U.S. Pat. No. 5,591,244, issued Jan. 7, 1997 and entitled "System for Removal of Noxious Fumes," discloses a fume control system for an asphalt kettle which passes fumes through a mobile filtration unit composed of a number of different filter elements. This approach suffers from a number of disadvantages. First, the system is relatively costly due to the need to include a plurality of specially manufactured filters. Second, the system is inconvenient to use due to the propensity of the filter units to become saturated with oils and solids associated with the fumes. Third, the system poses a possible safety hazard in that the oil-saturated filters can be ignited by the kettle.
Another recent design attempts to control kettle emissions by pulling fumes from the kettle through an combustion chamber housed in a stand-alone emission control unit. The FUMEGUARD.TM. asphalt fume elimination system, manufactured by Garlock Equipment Company of Minneapolis, Minn., includes a suction fan mounted downstream of the combustion chamber. This 150 cfm suction fan pulls the fumes into the combustion chamber. The FUMEGUARD.TM. includes a rotating loader mechanism similar to that described above in connection with U.S. Pat. No. 4,033,328.
The FUMEGUARD.TM. also includes an important safety feature. If the fan stops rotating, a negative pressure will arise within the kettle. This negative pressure can cause backdraft of ignited fumes from the combustion chamber into the melting chamber. To prevent ignition of the kettle under these conditions, the FUMEGUARD.TM. couples activation of the burner in the combustion chamber to rotation of the fan. Thus, in the event of a power failure or other problem affecting the fan the burner is automatically extinguished.
The FUMEGUARD.TM. design is viewed to have a number of disadvantages. First, the suction force provided by the 150 cfm can be inadequate to draw off kettle fumes when the hood of the device is raised to allow access to the melting chamber. In addition, positioning the suction fan downstream of the combustion chamber necessitates insertion of baffle structures between the burner flame and the suction fan, such that the baffle structures prevent the burner flame from being sucked into the fan. These baffles can interfere with thorough oxidation of the fumes by the afterburner, and increase the tendency of the flame in the afterburner to become extinguished. The baffles can also interfere with air flow through the afterburner chamber, limiting the effective suction force of the fan.
Moreover, these baffles can themselves provide an ignition source for fumes, effectively circumventing the safety precaution of linking the burner and fan. Specifically, if the fan fails and the burner is extinguished, the superheated baffles may still ignite fumes flowing back into the kettle.
Accordingly, there exists a need in the art for an emission control system for an asphalt kettle which overcomes the disadvantages present in the known art, and which provides safe and efficient removal of fumes even when the kettle lid is open to permit user access to the melting chamber.